1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) radio data communication system, particularly to a selection method of a handoff system between a soft handoff system and a hard handoff system.
2. Description of the Related Art
As is well known, as multiple access (MA) systems, there are known a time division multiple access (TDMA) system, a frequency division multiple access (FDMA) system and a code division multiple access (CDMA) system. Among the MA systems, the CDMA system is adopted in a mobile communication system in view of the fact that it is strong at noise and excellent in concealing performance. Further, in the mobile communication system of the CDMA system, when a mobile station is moved from one cell (a radio zone covered by a radio base station, sometimes called as a service area) to other cell, communication can be carried out with the both radio base stations covering the cells. Because the same frequency for radio communications in any cell is utilized in the CDMA system, and even when the mobile station moves between the cells, a reception frequency or a transmission frequency needs not to change. This is referred to as soft handoff or soft hand over in this technical field.
In contract thereto, according to the TDMA system or the FDMA system, only so-to-speak hard handoff (hard hand over) can be carried out. An interruption of communication occurs in the hard handoff when a communication channel is switched from one radio base station to another radio base station. Incidentally, caution is required to the fact that the CDMA system is not only able to carry out only soft handoff but also able to carry out both of soft handoff and hard handoff.
As described above, according to the mobile communication system of the CDMA system, even when the mobile station moves between the cells, the mobile station can always communicate with at least one radio base station with no momentary interruption. Therefrom, the mobile communication system of the CDMA system is provided with an advantage in which not only voice communication but also data communication can be carried out. In this specification, a mobile radio system of a CDMA system capable of dealing with also data communication is referred to as xe2x80x9cCDMA radio data communication systemxe2x80x9d.
Conventionally, the CDMA radio data communication system of this kind includes a radio base station control equipment (BSC) connected to a mobile switching center (MSC), a plurality of radio base stations (BTS) connected to the radio base station control equipment and a mobile station (MS) for carrying out data communication with the radio base stations while moving cells covered by the radio base stations. Further, the radio base station control equipment is also referred to as a base station control equipment or a radio control base station.
An explanation will be given of operation of a CDMA system in a hard handoff control in reference to FIG. 10. Here, an explanation will be given of the operation when a mobile station is switched from a state of carrying out data communication with an radio base station (hereinafter, referred to as xe2x80x9cold radio base stationxe2x80x9d) to data communication with other radio base station (hereinafter, referred to as xe2x80x9cnew radio base stationxe2x80x9d).
First, the mobile station carries out data communication with the old radio base station and the old radio base station carries out data communication with the base station control equipment. Assume that under the state, by movement of the mobile station to a vicinity of a boundary of a cell covered by the old radio base station, a reception electric field intensity of a pilot signal transmitted from the old radio base station at the mobile station is relatively weakened and by approach of the mobile station to a boundary of a cell covered by the new radio base station, a reception electric field intensity of a pilot signal transmitted from the new radio base station at the mobile station is relatively strengthened. In this case, the mobile station transmits a hard handoff request signal to the old radio base station. The old radio base station transmits the received hard handoff request signal to the base station control equipment.
In response to the hard handoff request signal, the base station control equipment carries out a channel assignment to the new radio base station. When the channel assignment can be confirmed, the new radio base station transmits a channel assignment confirmation signal to the base station control equipment. In response to the channel assignment confirmation signal, the base station control equipment transmits a handoff message to the mobile station via the old radio base station.
In response to the handoff message, the mobile station returns a handoff message response signal to the base station control equipment via the old radio base station. Thereafter, the mobile station transmits a preamble signal to the new radio base station. In response to the preamble signal, the new radio base station transmits establishment of synchronization to the base station control equipment. In response to the establishment of synchronization, the base station control equipment transmits a base station confirmation request signal to the mobile station via the new radio base station. In response to the base station confirmation request signal, the mobile station returns a base station confirmation response signal to the new radio base station.
Thereafter, the base station control equipment notifies channel release to the old radio base station. Further, the mobile station carries out data communication with the new radio base station and the new radio base station carries out data communication with the base station control equipment.
Next, an explanation will be given of operation of a CDMA system in a soft handoff control in reference to FIG. 11. Also in this case, an explanation will given of the operation when the mobile station is switched from a state of carrying out data communication with the old radio base station to data communication with the new radio base station.
First, the mobile station carries out data communication with the old radio base station and the old radio base station carries out data communication with the base station control equipment. Assume that under the state, by movement of the mobile station to a vicinity of a boundary of a cell covered by the old radio base station, a reception electric field intensity of a pilot signal transmitted from the old radio base station at the mobile station is relatively weakened and by approach of the mobile station to a boundary of a cell covered by the new radio base station, the reception electric field intensity of a pilot signal transmitted from the new radio base station at the mobile station is relatively strengthened. In this case, the mobile station transmits a first soft handoff request signal to the old radio base station. The old radio base station transmits the received first soft handoff request signal to the base station control equipment. The first soft handoff request signal is a request signal for assigning other channel since radio wave of a channel which has been used in the communication becomes weak.
In response to the first soft handoff request signal, the base station control equipment carries out channel assignment to the new radio base station. When the channel assignment can be confirmed, the new radio base station transmits a channel assignment confirmation signal to the base station control equipment. In response to the channel assignment confirmation signal, the base station control equipment transmits a handoff message to the mobile station via the old radio base station.
In response to the handoff message, the mobile station returns a handoff message response signal to the base station control equipment via the old radio base station. Thereby, xe2x80x9csoft handoff statexe2x80x9d, surrounded by a quadrangle of dotted lines in FIG. 11, is brought about. The mobile station carries out data communication with both of the old radio base station and the new radio base station and the old radio base station and the new radio base station carry out data communication with base station control equipment. In other words, the soft handoff state is a state in which communication is carried out by two channels.
Assume that thereafter, a reception electric field intensity of a pilot signal transmitted from the old radio base station at the mobile station is weakened to a degree by which data communication between the mobile station and the old radio base station becomes difficult. In this case, the mobile station transmits a second soft handoff request signal to the base station control equipment via both of the old radio base station and the new radio base station. The second soft handoff request signal is a request signal for releasing one channel since radio wave of a channel reaches a necessary level. In response to the second soft handoff request signal, the base station control equipment transmits a handoff message to the mobile station via both of the old radio base station and the new radio base station.
In response to the handoff message, the mobile station returns a handoff message response signal to both of the old radio base station and the new radio base station. At this time point, the mobile station is disposed on or outside of the boundary of the cell covered by the old radio base station and the old radio base station cannot receive a handoff message response signal. In contrast thereto, the mobile station enters within the boundary of the cell covered by the new radio base station and accordingly, the new radio base station can receive the handoff message response signal and the new radio base station transmits the received handoff message response signal to the base station control equipment.
In response to the handoff message response signal from the new radio base station, the base station control equipment notifies channel release to the old radio base station. In response to the notification of channel release, the old radio base station returns a channel release response signal to the base station control equipment. Further, the mobile station carries out data communication with the new radio base station and the new radio base station carries out data communication with the base station control equipment.
An explanation will be given of data communication (a selection method of the handoff operation) of a conventional CDMA radio data communication system described above in reference to FIG. 12.
When data communication is carried out in the conventional CDMA radio data communication system, the mobile station firstly makes a call connection request to a radio base station control equipment (radio control base station) via a radio base station (step E1). Successively, the mobile station notifies a handoff system (soft handoff) to be carried out to the radio control base station (step E2).
That is, according to the conventional CDMA radio data communication system, normally, data communication is carried out by using the soft handoff as shown in FIG. 11 as the handoff system. The hard handoff system as shown in FIG. 10 is used in a special case in which the radio base station is failed or the like.
In the meantime, there has been proposed a radio data communication system carrying out communication by using an agent which is not a CDMA radio data communication system. In this case, xe2x80x9cagentxe2x80x9d is referred to a concept which has been proposed as a man/machine interface of next generation in which all of operation necessary for executing a specific instruction is self-controllably executed on the side of a machine with no intermediary of manual operation. In other words, xe2x80x9cagentxe2x80x9d is referred to a software module capable of spontaneously determining and carrying out what is to be processed.
Further, an agent used in a mobile communication system is referred to as xe2x80x9cmobile agentxe2x80x9d. That is, the xe2x80x9cmobile agentxe2x80x9d is an agent for resolving a problem particular to a mobile communication environment. Further, almost all of agents used in the invention correspond to, (or are used by) communication application for eliminating wasteful communication such as data retransmission or the like by call disconnection caused in communication. Caution is required here that in each application, there is existed one agent in correspondence therewith (or used thereby).
For example, JP-A-9-231149 (hereinafter, referred to as prior art 1) has proposed a xe2x80x9cradio data communication systemxe2x80x9d resolving a problem in which the transmission efficiency is significantly deteriorated by retransmission or the like when channel quality is poor by a deterioration in a radio channel state or the like in the case of communication with no intermediary of an agent. According to the radio data communication system described in prior art 1, a first computer connected operationally to a first radio device and a second computer connected operationally to a second radio device communicate data via a radio channel. The respective computers are provided with interfaces of the radio devices to the computers between the first computer and the first radio device and between the second computer and the second radio device, the radio devices are provided with interface functions of the computers to the radio devices and there are arranged a first and a second agent operable self-controllably in accordance with a given situation.
Further, JP-A-9-214516 (hereinafter, referred to as prior art 2) is described with a xe2x80x9cmobile terminalxe2x80x9d capable of moving in a sub network where a current agent is not present so far as a home agent is present. The mobile terminal described in prior art 2 is provided with a location registering function, a location controlling function and a packet conversion function. According to the location registering function, the terminal is moved and connected to a different sub network and information of self location is informed to a home agent to thereby carry out a registering procedure and is periodically informed to a connected sub network. Further, the location controlling function controls the location information informed from a mobile terminal connected to the same sub network. Further, the packet conversion function detects whether a transmitted party is a mobile terminal or a fixed terminal and converts a packet in a pertinent style. The mobile terminal can move to a sub network where an agent is not present by controlling information necessary for movement of its own.
Further, there have been known various prior arts in respect of a mobile communication system of a CDMA system using soft hand over (soft handoff).
For example, according to JP-A-9-312869 (referred to as prior art 3), there is disclosed xe2x80x9ca method and a base station device as well as a mobile station device for executing xe2x80x9csoft hand over start/finish processingxe2x80x9d capable of optimizing a radio section capacity in both channels of a reverse channel and a forward channel. That is, according to prior art 3, when soft hand over for switching communication for a while from a first base station currently communicating to a second base station is carried out in accordance with movement of the mobile station, a trigger of start/finish of soft hand over in respect of a reverse communication channel from a mobile station to the base station and a trigger of start/finish of soft hand over in respect of a forward communication channel from the base station to the mobile station, are constituted by triggers which are set independently from each other.
Further, according to JP-A-10-13907 (hereinafter, referred to as prior art 4) there is disclosed xe2x80x9ca method of determining a kind of hand over in a CDMA mobile communication system and the CDMA mobile communication systemxe2x80x9d capable of determining a kind of an optimum hand over from a plurality of kinds of hand over. That is, according to prior art 4, the mobile station determines a start condition of a kind having the most moderate start condition in hand over of a plurality of kinds and when the determination is satisfied, the mobile station informs this to a base station. Further, the mobile station determines a start condition of hand over of a kind having the most moderate start condition other than hand over of specific kinds which cannot be executed and when the determination is satisfied, the mobile station informs this to the base station.
Further, according to JP-A-10-4580 (hereinafter, referred to as prior art 5) although not a mobile communication system of a CDMA system, there is disclosed xe2x80x9ca handoff communication systemxe2x80x9d capable of providing voice quality to a degree of that of conventional soft handoff even hard handoff having a simple system constitution is adopted. That is, according to prior art 5, a base station is installed with voiceless section detecting means for detecting a voiceless section of the communication, voiceless code data generating means for generating voiceless code data by detecting the voiceless section and transmitting data selecting means for selecting the voiceless code data in place of voice data at a front section of the voiceless section and supplying it to a transmitter. In the meantime, a mobile station is installed with handoff condition alleviating means for alleviating a handoff condition by lowering a threshold value of power of a reception signal necessary for switching the base station based on the voiceless code data transmitted from the base station.
However, according to the conventional CDMA radio data communication systems mentioned above, there poses the following problem. That is, the mobile station designates a soft handoff system as the handoff system so far as there is no abnormality of obstacle or the like in surrounding channels when a call connection is requested. Therefore, a plurality of channels are used in the handoff operation. Accordingly, a number of mobile stations capable of using a channel is limited.
Further, the above-described prior arts 1 through 5 only disclose technical thoughts which are quite different from that of an object of the present invention as respectively explained below although the prior arts more or less relate to the mobile communication system.
That is, prior art 1 only discloses technical thought with regard to the radio data communication system via agents and as not disclosed anything in respect of handoff control (selection) particular to the CDMA radio data communication system. Further, prior art 2 only discloses technical thought with regard to the mobile terminal capable of moving also to a sub network where the current agent is not present and does not disclose anything in respect of handoff control (selection) particular to the CDMA radio data communication system similar to the above-described prior art 1.
Also, either of prior arts 3 and 4 does not disclose anything in respect of handoff control (selection) in carrying out communication by using agents although the prior arts disclose technical thoughts with regard to a mobile communication system of a CDMA system. That is, prior art 3 only discloses technical thought in which triggers of start/finish of soft hand over are set to a reverse communication channel and a forward communication channel independently from each other. Further, prior art 4 only discloses technical thought in which an optimum hand over kind is determined from a plurality of kinds of hand over regardless of presence or absence of agents.
Further, prior art 5 only discloses the handoff communication system in which hard handoff having a simple system constitution is adopted and does not disclose anything in respect of handoff control (selection).
It is an object of the present invention to provide a CDMA radio data communication system in which handoff control with regard to a communication capable of continuing the communication by a function of a (mobile) agent even when the communication is interrupted in the midway, is constituted by hard handoff control.
The present invention adopts the following technical constitution in order to achieve the above-described object. That is, according to the present invention, a selection method of a handoff system in a CDMA radio data communication system characterized in a selection method of either of a hard handoff system and a soft handoff system as the handoff system, the method comprises: selecting the hard handoff system when the data communication is performed by using an agent function; and selecting the soft handoff system when an agent function is not available for the data communication.
More specifically, the method comprises: transmitting a call connection request signal, by a mobile station, including information of presence of the client side agent and a kind of the client side agent in correspondence with the application to be used in the communication, to the radio base station control equipment; searching presence of the server side agent corresponding to the client side agent in a agent server which is connected to the radio base station control equipment; selecting the hard handoff system when the server side agent corresponding to the client side agent is provided in a agent server; and selecting the soft handoff system when the server side agent corresponding to the client side agent is not provided in a agent server.
Further, a selection method of a handoff system in a CDMA radio data communication system characterized in a selection method of either of a hard handoff system and a soft handoff system as the handoff system, comprises: transmitting a call connection request signal, by a mobile station, including information of presence of the client side agent and a kind of the client side agent in correspondence with the application to be used in the communication, to the radio base station control equipment; searching presence of the server side agent corresponding to the client side agent in a agent server which is connected to the radio base station control equipment; returning the searching result of the server side agent; and determining the handoff system based on an availability of the server side agent in the agent server obtained by the searching result.
The hard handoff system is determined for selection of the handoff system to be performed when the server side agent is available, and the soft handoff is determined for selection of the handoff system to be performed when the server side agent is not available.
Also, selection of the handoff system may be determined by the mobile station, and informing the determining result to the radio base station control equipment.
On the other hand, selection of the handoff system may be determined by the radio base station control equipment, and informing the determining result to the mobile station.
Further more, according to the present invention, a selection method of a handoff system in a CDMA radio data communication system in which an agent server, connected to a radio base station control equipment and incorporating server side agents in correspondence with various applications, and a mobile station, incorporating a client side agent and carrying out a communication with radio base stations while moving in service areas by performing either of a hard handoff system and a soft handoff system, the method comprises: confirming an availability of the client side agent for an application to be used in the mobile station; transmitting a signal including information of the client side agent from the mobile station to the radio base station control equipment; confirming an availability of the server side agent, which is corresponding to the client side agent informed by the mobile station, in the agent server; informing the result of confirmation in the agent server to the mobile station; and determining the handoff system to be performed based on an availability of the server side agent in the agent server informed by the result of confirmation in the agent server.
In generating a call (or transmitting data) in data communication, the mobile station detects presence or absence of an agent function previously and informs the radio base station control equipment with presence or absence of an agent and a kind of an agent for application used along with the call connection request (or agent request). The radio base station control equipment detects whether there is an agent for application in correspondence with the agent server and informs the mobile station with a result of the detection. Thereby, the mobile station informs the radio base station control equipment that communication is carried out by the hard handoff system when the agent can be used and inform the radio base station control equipment that communication is carried out by the soft handoff system when the agent cannot be used.
In this way, according to the present invention, the hard handoff system is used as the handoff system in the case of data communication in which communication can be continued by the agent even if the call is interrupted and accordingly, the circuit can effectively be utilized. Because when the call is interrupted in the midway, the server (base station side) and the client (mobile station side) share information of transmitting data from where to where by how much and accordingly, successive data can be transmitted by reconnection from the mobile station.
Further, in the case in which communication is difficult to continue as in the case in which communication must be made again from the start when the call is disconnected, the soft handoff system is used as the handoff system and accordingly, loss of the call can be prevented.